Re: [PATCH 1/4] net: introduce helper sendpages_ok()

[Ofir Gal <ofir.gal@volumez.com>]

On 03/06/2024 10:18, Hannes Reinecke wrote:
> On 5/30/24 15:26, Ofir Gal wrote:
>> Network drivers are using sendpage_ok() to check the first page of an
>> iterator in order to disable MSG_SPLICE_PAGES. The iterator can
>> represent list of contiguous pages.
>>
>> When MSG_SPLICE_PAGES is enabled skb_splice_from_iter() is being used,
>> it requires all pages in the iterator to be sendable. Therefore it needs
>> to check that each page is sendable.
>>
>> The patch introduces a helper sendpages_ok(), it returns true if all the
>> contiguous pages are sendable.
>>
>> Drivers who want to send contiguous pages with MSG_SPLICE_PAGES may use
>> this helper to check whether the page list is OK. If the helper does not
>> return true, the driver should remove MSG_SPLICE_PAGES flag.
>>
>> Signed-off-by: Ofir Gal <ofir.gal@volumez.com>
>> ---
>>   include/linux/net.h | 20 ++++++++++++++++++++
>>   1 file changed, 20 insertions(+)
>>
>> diff --git a/include/linux/net.h b/include/linux/net.h
>> index 688320b79fcc..b33bdc3e2031 100644
>> --- a/include/linux/net.h
>> +++ b/include/linux/net.h
>> @@ -322,6 +322,26 @@ static inline bool sendpage_ok(struct page *page)
>>       return !PageSlab(page) && page_count(page) >= 1;
>>   }
>>   +/*
>> + * Check sendpage_ok on contiguous pages.
>> + */
>> +static inline bool sendpages_ok(struct page *page, size_t len, size_t offset)
>> +{
>> +    unsigned int pagecount;
>> +    size_t page_offset;
>> +    int k;
>> +
>> +    page = page + offset / PAGE_SIZE;
>> +    page_offset = offset % PAGE_SIZE;
>> +    pagecount = DIV_ROUND_UP(len + page_offset, PAGE_SIZE);
>> +
> Don't we miss the first page for offset > PAGE_SIZE?
> I'd rather check for all pages from 'page' up to (offset + len), just
> to be on the safe side.
We do, I copied the logic from iov_iter_extract_bvec_pages() to be
aligned with how skb_splice_from_iter() splits the pages.

I don't think we need to check a page we won't send, but I don't mind to
be on the safeside.

>> +    for (k = 0; k < pagecount; k++)
>> +        if (!sendpage_ok(page + k))
>> +            return false;
>> +
>> +    return true;
>> +}
>> +
>>   int kernel_sendmsg(struct socket *sock, struct msghdr *msg, struct kvec *vec,
>>              size_t num, size_t len);
>>   int kernel_sendmsg_locked(struct sock *sk, struct msghdr *msg,
>
> Cheers,
>
> Hannes